Stability - Hydrostatics & Trim — Most Asked Questions
11 questions asked about stability - hydrostatics & trim across the General and Fn 3 papers, heaviest first.
Learn the system first
-
1 Why tankers have less freeboard?
Also asked: “Why tanker has lesser freeboard”, “Why tanker hav lessfreeboard”
Answer
Tankers earn Type A reduced freeboard: tiny openings, high subdivision and gas-tight integrity let them load deeper safely.
- Type A ships — tankers, gas and chemical carriers — have small openings, high hatch coamings and tight subdivision.
- Dense liquid cargo in small tanks keeps permeability low and free surface designed for, unlike shifting dry bulk.
- Structure is built for a full head of dense cargo, so strength supports deeper loading.
- Dry-cargo Type B ships with big hatches and variable centres of gravity need the extra freeboard margin.
Surveyor follow-up: tanker loads a light clean product full to marks — stability better or worse than dense crude?
Full theory: Why Oil Tankers Have Less Freeboard -
2 Deck Line is a horizontal line measuring 300mm by 25mm. It passes through the upper surface of the freeboard
Answer
The deck line is the freeboard datum: 300 by 25 mm amidships with its top edge on the freeboard deck continued out to the shell.
- Placed amidships, 300 mm long and 25 mm wide, marking where the freeboard deck top meets the shell.
- Every load-line mark hangs from it — freeboard is measured down from this edge to the waterline.
- Punched in steel and painted for contrast so paint loss never hides the datum.
- Read with the disc and seasonal grid: datum first, allowed draft second.
Surveyor follow-up: point at the side shell and name the datum — then the line she floats at today.
Full theory: Load Line: Marks, Letters & Conventions -
3 S – Summer:- It is the basic freeboard line at the same level as the Plimsoll Line. Other load lines are marked based on this Summer freeboard line
Answer
Summer is the base load line at Plimsoll level: tropical above it, winter and winter North Atlantic below.
- Summer sits level with the disc bar — the reference every other mark offsets from.
- Tropical rides one forty-eighth of summer draft above summer; winter the same distance below.
- Winter North Atlantic sits 50 mm below winter for the harshest scheduled zone.
- Lines run 230 by 23 mm forward and aft of a vertical 540 mm from the disc centre, upper edges marking allowed depth.
Surveyor follow-up: summer draft 8 metres — how far above and below sit tropical and winter, and in what units?
Full theory: Load Line: Marks, Letters & Conventions -
4 m.calculate the percentage reserve buoyancy in DW of RD 1.020
Also asked: “4m.calculate the percentage reserve buoyancy in DW of RD 1.020”
Answer
Reserve buoyancy is the watertight volume above the waterline as a percentage: work the hydrostatics at the stated density, then divide.
- Read displacement, deadweight and waterplane area at the given draft from the hydrostatic tables.
- Correct for density 1.020 against salt 1.025: lighter water means deeper draft for the same mass — apply dock-water allowance.
- Reserve equals total watertight volume minus submerged volume, over total volume, times one hundred.
- State the answer against deadweight with the draft, density and TPC quoted beside it.
Surveyor follow-up: same ship moves from dock water to sea — draft up or down, and by what allowance?
Full theory: Ship Hydrostatics: Buoyancy, Displacement and Trim -
5 Deadweight,lightweight,displacement
Also asked: “Deadweight, light displacement, load displacement”
Answer
Displacement is the whole ship weighed in water; lightweight is the ship empty; deadweight is everything she may carry.
- Displacement at any draft comes off the hydrostatic curves — light plus dead equals the total floating weight.
- Lightweight is hull, machinery and outfit with nothing aboard, proved by inclining and lightweight survey.
- Deadweight is cargo, bunkers, water, stores and crew effects to reach load displacement.
- TPC, fresh-water allowance and density corrections move all three between salt, dock and loading conditions.
Surveyor follow-up: deadweight constant but draft deeper in dock water — density, TPC or allowance, and what number proves it?
Full theory: Ship Hydrostatics: Buoyancy, Displacement and Trim -
6 What is freeboard
Also asked: “define freeboard ,why”
Answer
Freeboard is working reserve buoyancy: freeboard-deck edge to waterline, marked amidships by Load Line rules.
- Measured down from the uppermost complete weather deck with permanent watertight closure below it.
- Basic values by length and cargo type, then corrected into zone marks for water density and season.
- More freeboard in winter North Atlantic, less in tropical fresh — the marks move with the water, not the ship.
- Guarantees range of stability and survival time after damage for muster and abandonment.
Surveyor follow-up: summer mark awash in tropical fresh with papers reading winter — overloaded or legal, and by which mark?
Full theory: Load Line, Freeboard and Tonnage Measurement -
7 Explain clearly the meaning of the term reserve buoyancy
Answer
Reserve buoyancy is spare floating power: enclosed watertight volume above the waterline waiting to lift extra weight.
- Only submerged volume buoys the ship now; everything sealed above the line stands by for immersion.
- Loading extra weight eats the reserve as the waterline climbs — freeboard is the reserve measured down.
- Quote as volume or as percentage of total enclosed volume for the surveyor’s worked example.
- Damage eats it fastest — breached reserve above the line decides whether she floats long enough to abandon.
Surveyor follow-up: reserve halved by deck cargo with draught steady — volume, weight or stability first to suffer?
Full theory: Load Line, Freeboard and Tonnage Measurement -
8 Shear strake, garboard strake, metacenter, rightning lever, center of buoyancy
Answer
Name five in one breath: top strake, bottom strake, balance point, lever arm and lift centre — steel, geometry and stability together.
- Garboard strake is the bottom shell strake beside the keel; sheer strake is the top side strake at the deck edge.
- Centre of buoyancy is the underwater volume’s centre lifting vertically through B as heel changes.
- Metacentre is where B’s vertical meets the centreline at small heels; GM measures initial stability.
- Righting lever GZ is GM times heel angle small — the moment that rights her or, if negative, rolls her.
Surveyor follow-up: G above M with the strakes perfect and B on centreline — stable, tender or capsizing, and why?
Full theory: Ship Hydrostatics: Buoyancy, Displacement and Trim -
9 Deck line to plimsoll marking- told freeboard
Answer
From the deck line down to the load line is the freeboard, and the disc through that gap is the ship’s whole summer allowance.
- The deck line is a horizontal mark 300 mm long and 25 mm wide that passes through the upper surface of the freeboard.
- The load line disc is 300 mm in diameter and 25 mm thick, with a horizontal line through it; the upper edge of that line is the summer salt water line.
- The vertical distance from the deck line to the load line is the summer freeboard — how much she can be loaded up before she is at her marks.
- The marks are punched on the hull so they stay visible if the paint fades, and repainted white or yellow on a dark background.
Surveyor follow-up: ship loaded to the disc with the deck line not even in sight — how much freeboard is left, and in what condition?
Full theory: Load Line: Marks, Letters & Conventions -
10 Difference between dwt and displacement
Answer
Displacement is the whole ship and everything aboard; deadweight is what she can still carry — the difference between them.
- Displacement is the actual weight of the ship and all aboard her at any moment, equal to the weight of water displaced.
- Light displacement is the ship at her designed light draught: hull, machinery, spares and water in the boilers.
- Deadweight is the weight of cargo, stores, bunkers and ballast — the difference between light and loaded displacement.
- So deadweight tonnage is a carrying capacity, while displacement is an instantaneous weight; both are quoted in tonnes but only one is a capacity.
Surveyor follow-up: a ship drawing deeper with no cargo loaded — is her deadweight up, her light condition up, or her draught simply wrong?
Full theory: Ship Hydrostatics: Buoyancy, Displacement and Trim -
11 Write briefly on a. Effect of density on draught and displacement
Answer
Draught follows displacement and water density together: same weight in denser water means less draught.
- Displacement is weight, and weight is displacement volume multiplied by water density, so the two cannot be moved independently.
- Entering denser water at the same displacement, the ship floats higher — draught falls; in fresh water she sits deeper.
- To keep the draught unchanged on entering denser water, displacement must increase by the ratio of the densities.
- Fresh water is about 1.000 against 1.026 for sea water, which is roughly a 2.5 per cent change of draught for the same load — not a small one.
Surveyor follow-up: ship floats 20 cm higher entering the Baltic — is that density, trim or a draught mark error?
Full theory: Ship Hydrostatics: Buoyancy, Displacement and Trim
Source: 123 written papers and surveyor oral banks, 24,613 entries, merged into 17,210 topics. Ticks are stored on this device only.